The present invention relates to antenna equipment for use with automobile, portable and cordless telephones and other mobile station radio units.
The mobile radio communication network has been steadily extended to meet a growing demand for daily use and cannot be accommodated in a single frequency band conventionally assigned thereto; now, it is assigned one more frequency band. It is desired, therefore, that every mobile station equipment be switchable between these two frequencies--this calls for an antenna equipment that resonates with two different frequencies. FIGS. 1 and 2 show prior art examples of such an antenna equipment adapted for resonance with two frequencies. In the example of FIG. 1 a resonance circuit 7 is provided at a midpoint in an antenna element 11 and has a resonance frequency different from that of the antenna element 11, and besides, a matching circuit 8 is connected between a feeder 14 and the antenna element 11 to match their impedances. In the example of FIG. 2 the matching circuit 8 between the antenna element 11 and the feeder 14 is adapted to resonate with two frequencies.
In the unit of FIG. 1 the matching circuit 8 is relatively simple in structure but the provision of the resonance circuit 7 at a midpoint in the antenna element 11 introduces complexity in the mechanical structure of the antenna equipment, and in general, the antenna element 11 readily becomes crimped at that portion. In the example of FIG. 2 the matching circuit 8 is complex in structure and the provision of such a complicated matching circuit 8 will increase the power loss or dissipation by the antenna circuit accordingly. Besides, in the prior art examples of FIGS. 1 and 2 an antenna current develops in an antenna housing 9 (indicated by a symbol of ground potential); consequently, in a radio unit of the type wherein the housing is held by hand, the current distribution varies with how the housing is held and with the movement of the human body, causing a change in the radiation characteristic of the antenna. Furthermore, the antenna characteristic itself is also affected by the shape and material of the housing and parts mounted thereon (such as a dial pad and a liquid crystal display screen).
In Japanese Patent Application Laid-Open No. 213303/87 there is disclosed an antenna equipment of a construction in which a coaxial line of a length .lambda./4 (.lambda. being the wavelength used) and a characteristic impedance Z.sub.0 is connected between the feeding point of a .lambda./2 rod antenna and a feeder of a characteristic impedance Zb, and the impedance Za of the antenna feeding point and the impedances Zb and Z.sub.0 of the above-mentioned feeder and coaxial line are selected such that .vertline.Z.sub.0 .vertline.=(ZaZb).sup.1/2, thereby implementing the intended impedance matching. The antenna equipment of the above construction is capable of achieving high gains for wavelengths which are integral multiples of .lambda./2; besides, since the impedance of the antenna feeding point is very high (infinite, theoretically), the antenna current flowing to the housing is limited, and consequently, the dependence of the antenna characteristic on the housing structure is low and even if the housing is held by hand, the radiation characteristic of the antenna does not appreciably change. With the above-described antenna structure, however, a second operating wavelength is limited to integral multiples of .lambda./2 in contrast to the first wavelength .lambda., and hence it cannot freely be chosen. Moreover, it is difficult to achieve high gains for two wave-lengths which are relatively close to each other within .lambda./2 in the frequency band assigned to the mobile radio communication.
The portable radio telephone utilizes, in many cases, a telescopic antenna equipment of the type that the antenna element is extended out of the unit housing during communication but housed in the housing while not in use. In Japanese Patent Application Laid-Open No. 170201/89, for example, there is disclosed an antenna of a construction in which a first rod (0.6 .lambda.) is received in a second rod (0.5.lambda.), which is received in a third rod, which is, in turn, disposed inside a metal pipe, thus forming a .lambda./2 long impedance matching coaxial line. Such a telescopic antenna equipment facilitates carrying the radio telephone while not in use for communication, but the portable radio telephone needs to be held in the wait-receive mode in which to continue receiving electric waves from a base station at all times while not in use for communication, too. Hence, when the antenna element is retracted into and housed in the unit housing in the above-mentioned wait-receive mode, the impedance characteristic of the antenna will change, resulting in extreme reduction of its gain for received waves. In this instance, if the housing is made of metal, the sensitivity of the antenna will go down to substantially zero since it is covered with metal. Thus, it is impossible, in principle, to use such an antenna in its retracted state in the radio telephone that must be held in the wait-receive mode during the non-communication period. On the other hand, a diversity antenna requires two antenna elements, and hence is inevitably bulky.